A control valve is a type of valve used to control your process. There are many different types of control valves each serving a different purpose. Some types of control valves are Pressure Control Valves, Temperature Control Valve, Level Control Valve and Dump Valves.
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Characteristics
Fast Opening
The fast opening characteristic valve plug will give a large change in flowrate for a small valve lift from the closed position. Typically fast opening valves are used for on/off type applications.
Equal Percentage
An Equal Percentage characteristic has a flow capacity that increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv. This provides excellent rangeability and the control resolution at small increments of stem travel.
Linear
The linear characteristic valve plug is shaped so that the flowrate is directly proportional to the valve lift, at a constant differential pressure. A linear valve achieves this by having a linear relationship between the valve lift and the orifice area. For example, if the valve stem position is 25% open, 25% of the flow rate is allowed to pass at a constant differential pressure.
Leakage Classifications
There are many standards for classification of valve leakage rates depending on the industry and valve type (cryogenic, plastic, ect.) In the oil and gas industry, the three most used standards are API 598 - Valve Inspection & Testing, ANSI FCI 70-2 - Control Valve Seat Leakage and MSS-SP-61 - Pressure Testing of Steel Valves.
API Std 598
- View API Std 598
All valves built to the various API standards are required to meet API-598 leakage criteria prior to shipment from the manufacturer or supplier. The allowable rate for leakage of test fluid past the seats, for the duration of the tests, is shown below.
| Nominal Valve Size | All Resilient Seated Valves | Metal Seated Valves (Liquid Test) | Metal Seated Valves (Gas Test) |
|---|---|---|---|
| 2" and under | 0 | 0 drops/min | 0 bubbles/min |
| 2.5" - 6" | 0 | 12 drops/min | 24 bubbles/min |
| 8" - 12" | 0 | 20 drops/min | 40 bubbles/min |
| 14" and up | 0 | 28 drops/min | 56 bubbles/min |
Note:
- One (1) milliliter is considered equivalent to 16 drops.
- There shall be no leakage for the minimum specified test duration. For liquid test, 0 drop means no visible leakage per minimum specified test duration. For gas test, 0 bubble means less then 1 bubble per minimum specified test duration.
ANSI FCI 70-2
ANSI FCI 70-2 supercedes ANSI B16.104 and specifies six different seat leakage classifications, Class I through Class VI.
| Leakage Class Designation | Maximum Leakage Allowable | Test Medium | Test Pressure & Temperature |
|---|---|---|---|
| Class I - Dust Tight | x | x | x |
| Class II | 0.5% of rated capacity at full travel | Air / Water | 50 psig or maximum differential pressure, whichever is lower. Temperature between 50-125°F (10-52°C) |
| Class III | 0.1% of rated capacity of full travel | Air / Water | 50 psig or maximum differential pressure, whichever is lower. Temperature between 50-125°F (10-52°C) |
| Class IV - Metal to Metal | 0.01% of rated capacity of full travel | Air / Water | 50 psig or maximum differential pressure, whichever is lower. Temperature between 50-125°F (10-52°C) |
| Class V | 0.0005 ml per minute per inch of port diameter per psi differential | Water | Service DP at 50° to 125°F (10° to 52°C) As long as the maximum service pressure across valve does not exceed the ANSI body rating |
| Class VI - Bubble Tight | Not to exceed amounts shown in the table below | Air or Nitrogen | Service DP at 50° to 125°F (10° to 52°C) As long as the maximum service pressure across valve does not exceed the ANSI body rating |
Class VI Bubble Tight Leakage Rates
| Port Size
(Inches) |
Port Size
(mm) |
Bubbles per Minute | ml per Minute |
|---|---|---|---|
| 1 | 25 | 1 | 0.15 |
| 1.5 | 38 | 2 | 0.3 |
| 2 | 51 | 3 | 0.45 |
| 2.5 | 64 | 4 | 0.6 |
| 3 | 76 | 6 | 0.9 |
| 4 | 102 | 11 | 1.7 |
| 6 | 152 | 27 | 4 |
| 8 | 203 | 45 | 6.75 |
